Interpretive Summary: Corn rootworms eat corn causing economic damage. Many cultural and chemical management methods have failed to control these adaptive pests. Today, genetically modified corn producing rootworm-toxic proteins are widely used to reduce corn rootworm populations. There is concern corn rootworms will evolve resistance to the toxic proteins, and managing resistance is dependent upon susceptible beetles mating with resistant beetles. But, females may choose to mate with males providing the largest nuptial gift via the spermatophore, which often relates directly to body size. The relationship between body size and resistance is unknown in corn rootworms, and there is little known about the mating behavior and reproductive biology of corn rootworms. We examined spermatophore size of the northern corn rootworm in relation to body size of males and females. Copulating males lost significantly more weight than did non-copulating males, whereas copulating females gained significantly more weight than did non-copulating females. For copulating pairs, male and female weight before pairing correlated positively suggesting assortative mating. The percent change in weight of mated males correlated positively with male weight before pairing and negatively with pair duration. In contrast, the percent change in female weight correlated negatively with both female weight before pairing and pair duration. If assortative mating is occurring with respect to resistance to protein toxins, it could reduce the time to crop failure. This information could benefit scientists writing computer models on insect resistance management.

Technical Abstract:
Corn rootworms (Diabrotica spp.) can inflict serious damage to maize (Zea mays L.) and have evolved resistance to cultural and chemical management methods. Currently genetically modified maize producing rootworm-toxic proteins derived from the bacterium, Bacillus thuringiensis Berliner, is extensively used to manage corn rootworm populations. Management of resistance to Bt maize is contingent upon Bt susceptible beetles mating with Bt resistant beetles. Females may choose to mate with males providing the largest nuptial gift via the spermatophore, which may correlate with fitness related characters such as body size. Here, we examined spermatophore size of Diabrotica barberi Smith & Lawrence in relation to body size of males and females. Virgin males and females of known ages were weighed before and after pairing. Pairs were noted as copulating or not copulating. Copulating males lost significantly more weight than did non-copulating males, whereas copulating females gained significantly more weight than did non-copulating females. For copulating pairs, male and female weight before pairing correlated positively. The percent change in weight of mated males correlated positively with male weight before pairing and negatively with pair duration. In contrast, the percent change in female weight correlated negatively with both female weight before pairing and pair duration. If assortative mating is occurring with respect to Bt resistance, it could hasten the time to crop failure even with high dose toxins.